Cosmetic antiperspirant formulations are known and available to the public in several different physical forms for application using the corresponding type of applicator, including dispensers for powder mixes, foams, gelled or thickened liquids, liquids of low viscosity that can be sprayed, aerosol formulations, creams, soft solids and sticks. The preferred choice of physical form can often depend on the history of product, and local preferences, which may themselves vary over time as fashions change. One physical form which has been popular especially in North America for antiperspirant and deodorant compositions during the last twenty years is that of sticks. The term “stick” herein is employed in its natural meaning, that is to say a material that is firm to the touch, is often in the shape of a rod or bar and commonly is housed in a container comprising a barrel having an open end and an opposed piston which can be slid up the barrel to expel the stick, which retains its shape and integrity during its expulsion.
Cosmetic antiperspirant sticks typically comprise an antiperspirant active that is dissolved or suspended in a cosmetically acceptable carrier material of which at least a fraction is a cosmetically acceptable water-immiscible oil. In one highly desirable class of cosmetic sticks, the carrier material comprises either no polar liquid or no more than the proportion that can form a single liquid phase with the water-immiscible oil or oil mixture.
One class of material that has hitherto been proposed for solidifying water-immiscible oils comprises non-polymeric fibre-forming structurants. A number of such structurants comprise alkyl ester derivatives of certain saccharides, such as maltose or particularly cellobiose, and others comprise N-acyl amido derivatives of aminoacids, di- or tri-carboxylic acids or cyclohexane. The present invention is directed particularly to compositions in which a continuous phase comprising a water-immiscible oil is solidified with one or more N-acyl amido derivatives of aminoacids.
Many N-acyl amido derivatives of aminoacids that are suitable for solidifying cosmetically-acceptable oils to a greater or lesser extent have been described by Ajinomoto Co Ltd in U.S. Pat. No. 3,969,087, including in particular derivatives of glutamic acid or aspartic acid. The derivative disclosed therein that was apparently the most preferred by Ajinomoto was N-lauroylglutamic acid, -di-n-butylamide, as also indicated by the fact that for many years, it was the only such material that was commercially available from them (trade name GP-1).
GP-1 structurant has been disclosed for use or used in structuring water-immiscible oils in cosmetic sticks, but often not by itself and instead in combination with one or more structurants, for example providing the minor weight proportion of the structurant mixture. Thus, for example Hofrichter et al (Procter & Gamble) in U.S. Pat. No. 5,650,144, U.S. Pat. No. 5,591,424 and U.S. Pat. No. 5,429,816 describe the formation of sticks in which a cosmetic oil is solidified with a mixture of a major proportion of 12-hydroxystearic acid or related compounds (primary gellant) and a minor proportion of an N-acyl aminoacid amide (secondary gellant), exemplifying GP-1 and related N-acyl glutamic acid di-amides in a weight proportion to 12-HSA of 2:6. The combination of hydroxystearic acid and N-acyl aminoacid amides gellants described in the Hofrichter patents supra can be processed under acceptable processing conditions, which is a very desirable attribute. However, the combination of the N-acyl aminoacid amides with hydroxy-stearic acid as primary structurant has a second consequence. Such combinations when made prior to this invention were opaque, rather than translucent.
In the course of investigations leading to the instant invention, it was found that although sticks can indeed be made using N-acyl aminoacid amides like GP-1 as gellant, the resultant product was comparatively soft when made, depositing a “wet” oily film on skin when applied topically. Such a feel is disliked by consumers. Such disadvantageous properties tended to become worse during storage of the product.
The comparative softness of such products has been recognised by Ajinomoto themselves. More recently, in USA-2002/0159961, Ajinomoto has described a selection of N-acyl amido derivatives of aminoacids from within the overall ranges described in U.S. Pat. No. 3,969,087. In this selection, the alkyl group R3 in the N-acyl substituent —CO—R3 is characterised by containing from 7 to 10 carbon atoms, and may be branched. The '961 specification discloses that the new selection of aminoacid derivatives can be employed to gel non-polar organic liquids to produce harder gels. The '961 specification also discloses the formation of antiperspirant compositions gelled by a representative member of their selection or mixed with GP-1, but once again their selected gellant in which the acyl N-substituent is branched is employed as the minor gellant in combination with hydroxystearic acid as primary gellant (weight ratio of 2:7). Such products are again opaque.
One of the problems faced in the course of devising the instant invention is that of the temperature at which a water-immiscible cosmetic oil gels when employing N-acyl aminoacid amide gellant having a branched N-acyl substituent compared with the same amount of a like gellant having a linear N-acyl substituent. For otherwise identical compositions, the gellant having the branched N-acyl substituent gels such a composition at a significantly higher temperature, for example a difference of over 20° C. A mixture of a gellant such as an N-acyl aminoacid amide and a carrier oil needs to be heated to substantially above its gelation temperature before the gellant dissolves, and it is commonly impractical for antiperspirant or deodorant compositions to be heated to amide dissolution temperatures, so that, in practice, it is impractical to redissolve the gellant by heating such a composition once it has gelled and it accordingly remains gelled. Consequently, it is inherently disadvantageous to employ a gellant that gels the composition at a significantly higher temperature, such as to above the boiling point of water. An elevated gelation temperature introduces a substantial risk that the composition would be gelled before it has been cooled to a temperature at which an active constituent or a temperature sensitive constituent can be introduced, or that the very act of introduction of the active constituting a significant proportion of the overall composition would lower the composition temperature rapidly below the oil gelation temperature, rendering subsequent operations extremely difficult if not impossible on a bulk scale, such as filling of product dispensers.
There is further potentially serious complication in the context of seeking to make a translucent composition. If the temperature of the mixture containing a suspended antiperspirant active is taken above 100° C., there is an increased risk of water evaporating off from the antiperspirant. This has the effect of inevitably altering the refractive index of the particulate solid. The very nature of the process means that it is not readily controlled and the extent of loss is not easily predicted. Translucent compositions containing commonly available particulate substances are, in practice, often obtained by refractive index matching of carrier and suspended material. Hence, if the formulation is prepared under conditions at which water evaporates off, refractive index matching becomes a matter of chance rather than control. Also the risk is increased that different antiperspirant particles lose water to a differing extent, increasing the refractive index spread of the suspended material, and thereby inevitably meaning that at least some suspended particles will not be refractive index matched causing opaqueness.